A commonly encountered objective in the operation of FEDs, such as field emission displays, is control of the spot size of an electron beam. With respect to the operation of a field emission display, a controlled spot size provides improved resolution, brightness, and, for a color display, improved color purity.
FIG.1 illustrates, in cross-section, a prior art FED 100 having a planar focusing electrode 128 for controlling the spot size of an electron beam 130. FED 100 includes a cathode plate 110 and an anode plate 111, which opposes cathode plate 110.
Cathode plate 110 has a substrate 114. First, second, and third cathodes 115, 116, 117 are disposed upon substrate 114. A first dielectric layer 118 is formed upon cathodes 115, 116, 117 and includes a plurality of emitter wells 121. In each of emitter wells 121 is formed an electron emitter 120. Cathode plate 110 further includes a gate electrode 125 disposed upon first dielectric layer 118 and circumscribing each of emitter wells 121. A second dielectric layer 126 is disposed upon gate electrode 125. A focusing electrode is disposed upon second dielectric layer 126.
Anode plate 111 includes a transparent substrate 113, upon which is formed an anode 112. A red phosphor 132, a green phosphor 134, and a blue phosphor 136 are disposed upon anode 112. Electron emitters 120 that are connected to first cathode 115 oppose red phosphor 132; electron emitters 120 that are connected to second cathode 116 oppose green phosphor 134; electron emitters 120 that are connected to third cathode 117 oppose blue phosphor 136.
FED 100 is operated by applying potentials at first, second, and third cathodes 115, 116, 117 and to gate electrode 125 for causing selective emission from electron emitters 120. A potential is also applied to planar focusing electrode 128. The potential at planar focusing electrode 128 is selected to collimate electron beam 130 and to result in a controlled spot size at red, green, and blue phosphors 132, 134, 136.
Prior art FED 100 requires additional processing steps to form second dielectric layer 126 and planar focusing electrode 128. It would be desirable to reduce the number of processing steps in the fabrication of an FED, while retaining the control over the spot size of the electron beams.
FIG. 2 is a cross-sectional view of another prior art FED 200, which has multiple, selectively addressable anodes. Elements of FED 200 that are the same as those of FED 100 are similarly referenced, beginning with a "2". In contrast to FED 100, which has a single-anode configuration, FED 200 has a first anode 212, a second anode 238, and a third anode 240 within each pixel. A red phosphor 232 is connected to first anode 212; a green phosphor 234 is connected to second anode 238; and a blue phosphor 236 is connected to third anode 240.
Each pixel of FED 200 has one cathode. The cathode is used to cause emission from a plurality of electron emitters 220, which are in the given pixel. Illustrated in FIG. 2 is a first pixel 242 having a first cathode 216. Further illustrated is a portion of a second pixel 244 having a second cathode 217.
All of electron emitters 220 of a given pixel are caused to emit regardless of which of phosphors 232, 234, 236 are to be activated. Color selectivity is achieved by selectively addressing first, second, and/or third anodes 212, 238, 240. An electron beam 230 from each of electron emitters 220 is attracted toward the selected phosphor(s) by applying an attracting potential to the anode(s) connected thereto. In this manner, electron emitters 220, which do not directly oppose the selected phosphor, are also utilized to activate the selected phosphor.
A shortcoming of prior art FED 200 is that portions of electron beams 230 can be attracted toward phosphors of adjacent pixels. As show in FIG. 2, electrons from electron beam 230, which are intended to be directed toward green phosphor 234, can undesirably be diverted toward red phosphor 232 of an adjacent pixel, if first anode 212 of the adjacent pixel is simultaneously addressed. This phenomenon results is the loss of color purity for FED 200.
A further shortcoming of FED 200 is that the multiple anode configuration is not practical for operation at high anode voltages, such as, for example, greater than about 600 volts. This is due to the greater power requirements for switching at high voltages and the increased risk of electrical arcing between the anodes. It would be desirable, therefore, to provide improved color purity of an FED, while simultaneously avoiding switching a high anode voltage.
FIG. 3 is a cross-sectional view of yet another prior art FED 300 having a phosphor area, A.sub.P, that is greater than an emitter area, A.sub.E. Elements of FED 300 that are the same as those of FED 100 are similarly referenced, beginning with a "3". FIG. 3 depicts a sub-pixel of FED 300. A plurality of electron emitters 320 oppose a red phosphor 332. In the operation of FED 300, no attempt is made to focus an electron beam 330 subsequent to its emission from one of electron emitters 320. Thus, a spot size, S, of electron beam 330 at red phosphor 332 is relatively large. In order to confine electron beam 330 to receipt at red phosphor 332, emitter area, A.sub.E, is made smaller than phosphor area, A.sub.P.
FED 300 suffers from the drawback of a limited emitter area. A reduced emitter area can reduce the overall current for stimulating the phosphor and thus reduces the brightness. An increased phosphor area are can result in poor resolution for FED 300.
Accordingly, there exists a need for an improved FED having a focusing capability that simplifies the fabrication of FEDs and that further improves brightness, color purity, and resolution over the prior art.